NATURE OF A CARDIAC CONTRACTION. 109 



Special Actions Of Cardiac Poisons. The complicated actions of various 

 poisons upon the heart, have led observers to suppose that there are various intra- 

 cardiac mechanisms on which these substances may act. Besides the muscular 

 fibres of the heart and its automatic ganglia, some toxicologists assume that there 

 are inhibitory ganglia into which the inhibitory fibres of the vagus pass, and 

 accelerator ganglia, which are connected with the accelerating nerve-fibres of the 

 heart. Both the inhibitory and accelerator ganglia are connected with the automatic 

 ganglia by conducting channels. 



Muscarin stimulates permanently the inhibitory ganglia, so that the heart 

 stands still (Schmiedeberg and Koppe). As atropin and daturin paralyse these 

 ganglia, the stand-still of the heart brought about by muscarin may be set aside by 

 atropin. [If a frog's heart be excised and placed in a watch-glass, and a few drops 

 of a very dilute solution of muscarin be placed on it with a pipette, it ceases to 

 beat within a few minutes, and will not beat again. If, however, the muscarin be 

 removed, and a solution of atropine applied to the heart, it will resume its contrac- 

 tions after a short time.] Physostigmin [Calabar bean] excites the energy of the 

 cardiac muscle to such an extent, that stimulation of the vagus no longer causes 

 the heart to stand still. lodine-aldehyd, chloroform, and chloral-hydrate paralyse 

 the automatic ganglia. The heart stands still, and it cannot be made to contract 

 again by atropine. The cardiac muscle itself remains excitable after the action of 

 muscarin and iodine -aldehyd. so that if it be stimulated it contracts. [According 

 to Gaskell, antiarin and digitalin solutions produce an alteration in the condition 

 of the muscular tissue of the apex of the heart of the same nature as that pro- 

 duced by the action of very dilute alkali solution, while the action of a blood solution 

 containing muscarin closely resembles that of a dilute acid solution ( 65).] 



[Nature of a Cardiac Contraction. The question as to whether 

 this is a simple contraction or a compound tetanic contraction, has been 

 much discussed. This much is certain, that the systolic contraction 

 of the heart is of very much longer duration (8 to 10 times) than the 

 contraction of a skeletal muscle produced by stimulation of its motor 

 nerve. When the sciatic nerve of a nerve-muscle preparation (" rheo- 

 scopic limb") is adjusted upon a contracting heart, a simple secondary 

 twitch of the limb, and not a tetanic spasm, is produced when the 

 heart (auricle or ventricle) contracts. This of itself is not sufficient 

 proof that the systole is a simple spasm, for tetanus of a muscle does 

 not in all cases give rise to secondary tetanus in the leg of a rheoscopic 

 limb. Thus, a simple " initial " contraction occurs, when the nerve is 

 applied to a muscle tetanised by the action of strychnia, and the con- 

 tracted diaphragm gives a similar result. The question as to whether 

 the heart can be tetanised, has been answered in the negative, and as 

 yet it has not been shown that the heart can be tetanised in the same 

 way that a skeletal muscle is tetanised.] 



The peripheral or extra-cardiac nerves will be discussed in connec- 

 tion with the Nervous System. 



59. The Cardio-Pneumatic Movement. 



As the heart within the thorax occupies a smaller space during the 

 systole than during the diastole, it follows that when the glottis is open, 



